Tree houses are for little kids—or so we thought. The fort designs we remember are becoming architectural wonders that can rival most homes. Not just for play dates, these sky-high structures are being used for single family homes, guest bedrooms, home offices, even romantic retreats.

Nut House (Dusseldorf, Germany)

Deep inside a walnut forest, we found this charming modern tree house that's shaped like the nut itself. Stilts support most of the weight, while the terrace is strapped to a nearby tree. Because of the height and large windows, we imagine that once we're inside, it'll feel like there's nothing but air between the leaves and us. It's used as a playroom for the owner's three children, and as an occasional guest room.

Tree Canopy (Virtual)

We knew this wild rendering was too good to be true—but someday it might be. Inspired by an African acacia tree, the people at 51% Studios envisioned it as a treetop retreat for safarists who track wild animals in the savannah. It blends in with its surroundings, while offering safety from harsh conditions and enough space in which to sleep and examine the landscape. If it is built—we'll be on the first plane to the Serengeti.

The Spiral House (Rambouillet Forest, France)

We're not sure if we are more impressed by the tree house itself or the staircase to get to it. For a dramatic and Earth-conscious construction, the builders chose to wind the spiral staircase around the trunk instead of chopping branches.

Treehouse (Ontario, Canada)

Not just the occasional getaway, this year-round home is precariously attached to four trees. During the day, the latticed walls act like a tree canopy, filtering light into the rooms. By night, it resembles a lantern glowing above the trees near Lake Muskoka. There's a first floor bedroom inside, and two upper floors that are open to the elements. We commend this modern design for its beauty and bravery.

Sophie's Dream House (United Kingdom)

In 2002, the Daily Telegraph, a U.K. newspaper, called for the nation's children to submit their best tree house designs. Sophie Hughes, who was 11 years old at the time, won the contest after beating out 800 entrants. As her prize, builders from PearTree Limited descended upon her home to recreate her design in the garden. The little house has acorn-shaped windows and a delightfully creaky bridge, plus a pulley that her mom uses to send up food in a basket.

Lifepod

The people at Kyu Che Studio wanted to design a space that fits in any environment. They came up with the Lifepod, a small, oval-shaped cabin that can be moved and assembled almost anywhere. It can be used as a small office or guest room, and can accommodate two people for several days. The structure can be fixed to any terrain, or elevated high into the air by a system of adjustable steel legs and cables anchored to nearby trees.

Riverside Tree House (United Kingdom)

A colorful tree house tucked between the vines of a willow tree, perched over a river—what could be more whimsical? When the weather's fine, the owners lounge on the veranda, while the little ones throw their fishing poles into the water. Meanwhile, the three eldest boys may decide to jump off the roof, making a splash into the river.

The Learning Tree (Geneva, Switzerland)

The parent of four little girls commissioned the French design company, La Cabane Perchee, to fit two pretty houses and a terrace on one large, imposing oak. To help the girls learn about nature, the builders put in a table engraved with the names and images of the surrounding trees.

Treetent

Built by renowned artist Dre Wapenaar, this tree pod was originally designed as a refuge for British activists during deforestation protests. It can sleep up to two adult protesters or four pacifist Children.

Sky High Tree House (Saleve Mountain, France)

Teetering at the top of 130-foot Austrian pine, this house looks like it has no visible means of support (a hidden ring bears all of the weight). It probably takes forever to climb up the spiral staircase, but we envy the spectacular views of Lake Geneva that the owner can see from his bedroom window.

Home Office Tree House (United Kingdom)

If we could work at this enchanted spot, we'd never leave our desks. It lies in the outskirts of London, where the lucky owner uses it as a home office. He has electricity to power up his computer, and a deck to use when the sun shines. When it's winter, the house can be heated to keep it cozy and warm.

Cedar Spire (Fife, Scotland)

A fairy-tale castle sits on the grounds of a grand Scottish estate. Its majestic details include: stained-glass windows, conical roofs and a balcony that's perfect for playing Rapunzel. A construction this whimsical keeps the kids occupied, so the adults are free to host parties on the neighboring deck.